Multiple views of a prior art x-ray inspection system, to which the present invention may usefully be applied, are depicted in FIGS. 1A-1D. All systems for security inspection from a moving platform inspecting a vehicle using x-rays transmitted through an inspected vehicle have either required a fixed portal or gantry, or else have employed a ponderous boom or other deployment mechanism, either of which requires a specially designed vehicle for carrying such a mechanism to each inspection venue. Salient components of such mobile inspection systems are described with reference to the numerals designated in FIGS. 1A and 1B. In prior art mobile x-ray inspection system 100, a plurality of detector modules 102 are arrayed in a pontine detector array 104. Detector elements that comprise each of the modules 102 receive radiation directed from an x-ray source 106.
Source 106 emits penetrating radiation, typically x-rays, that are transmitted through inspected object 124 (otherwise referred to herein as a “target”) and detected by the detector elements of detector array 104. Support of detector array 104 relative to conveyance 110 (otherwise referred to herein as a “moving platform” or an “inspection platform”) is provided by boom 121 and mast 122, jointly comprising a structure that may be referred to herein as a “pontine structure.” Boom 121 and mast 122 may be adapted for deployment during inspection operation, and stowed for on-road travel.
X-ray inspection system 100 is typically entirely integral with conveyance 110. Conveyance 110 may be a truck, capable of self-propulsion on, or off, roads, or conveyance 110 may also be a trailer that may be hauled by a truck or other self-propelled tractor. X-ray inspection system 100 may also be operated in a “portal” mode, where the system is stationary and inspected object 124 (which may be a vehicle, or a trailer) moves past it. In other cases, x-ray inspection system 100 may move past an inspected object.
Security inspection based on emission of x-rays from moving platform 110 and detection of x-rays traversing inspected object 124 is limited with respect to the speed of the moving platform 110 relative to the inspected object 124. Inspected object 124 may be a vehicle, for example. The reason for stringent speed requirements is that x-rays must dwell on each inspected cross-section of the target 124 for a duration of time that achieves an adequate ratio of signal to noise in the detected transmission signal. If the relative speed varies significantly, the sensitivity of detection will vary across regions of the inspected object, with faster scanning corresponding to lower sensitivity.
Therefore, motion of the inspection platform 110 must be governed in some manner, so as not to exceed a useful scanning speed. Prior art x-ray inspection systems 100 employed myriad design approaches. In some cases, the motion of inspection platform 110 was governed open-loop, with a driver required to maintain a specified scanning speed. In other cases, an independent motive system was added into or onto the inspection platform 110, typically a truck chassis. Examples have included counter-rotating drum systems that use friction applied to the truck tire, deployable systems that contacted the ground (tag axles, helper drives), drive shaft systems that rotated the vehicle's drive shaft with the transmission in neutral, or transfer case systems that interrupted the standard driveline with additional electric or hydraulic motors. Some of these systems used energy provided by the truck engine converted either by a power take-off (PTO) or through a transfer case. Other systems have been powered by electricity from an onboard generator or a shore power connection.
Unfortunately, each of the prior art systems for limiting scanning speed has its own drawbacks. These include reliance on driver judgment, complexity and attendant maintenance requirements, the requirement of additional external sources of power, and the need for finely-tuned control and feedback systems.
It would be advantageous to inspect vehicles without the complexity overhead of existing systems that are invariably employed to limit scanning speeds.